ICES Ecosystem Overviews describe the current state of ecosystems, identify the main human pressures, and explain how these pressures affect key ecosystem components. As a series, the overviews highlight the capacity of ICES to advance and shape ecosystem understanding.
The latest ecosystem overview to be published has been produced for the Baltic Sea. The Baltic Sea ecoregion lies in northern Europe, comprising a semi-enclosed shallow sea bordered by nine countries. One of the largest brackish water bodies in the world, it is characterized by strong vertical stratification and temperature and salinity that decrease as you move from the southwest of the region to the northernmost parts. This environmental gradient shapes the biological system from relatively diverse marine food webs in the southwest to less diverse freshwater ones in the north.
Lauréne Pécuchet, DTU Aqua and a member of ICES/HELCOM Working Group on Integrated Assessments of the Baltic Sea (WGIAB), has highlighted some of the key pressures in the ecoregion.
"One of the most important human pressures in the region is nutrient and organic enrichment. Though the overall loading of nutrients has decreased, the annual nutrient input still exceeds agreed goals in many regions and the nutrient concentrations in the water column and sequestered in sediments remain high. These high nutrient concentrations lead to eutrophication, visible through increases in blue-green algal blooms and excessive filamentous algal growth in many coastal areas. Eutrophication, combined with a reduced frequency of oxygen-rich water inflows from the North Sea, has resulted in large deep-water areas with poor or no oxygen. In these anoxic deep basins, benthic life is scarce or absent below the halocline."
"Fishing is an important human activity in the ecoregion. The principal commercial species targeted are cod (Gadus morhua), herring (Clupea harengus), and sprat (Sprattus sprattus), which constitute about 95% of the total catch. The overall fishing effort in the ecoregion fell by approximately 50% from 2004 to 2012 but many stocks are still experiencing overfishing (fished at greater than FMSY).
The biomass of western Baltic cod has been below the limit reference point for at least ten years while eastern Baltic cod is decreasing and its size structure and condition factor have deteriorated markedly without signs of improvement.
Seabird species eating sprat and herring have increased in number, while several that feed on the benthos are decreasing, possibly partly caused by bycatch in static net fisheries."
"Climate change has already influenced aspects of the Baltic Sea ecosystem with the annual mean sea-surface temperature increasing by up to 1°C per decade. The annual maximum ice extent has decreased and the length of the ice season has become shorter.
Large-scale alterations in the hydrography, biogeochemistry, and physical properties of the Baltic Sea are expected during this century, including long-term changes in temperature, ice cover, salinity, oxygen, nutrient concentrations, and primary production. These will likely impact phytoplankton, zooplankton, benthos, fish, seabirds, and habitats.
Such impacts are already observed in coastal fish communities with an increasing abundances of fish from the carp family (Cyprinidae) and decreases in piscivorous fish in many areas as a response to environmental change."
The Baltic Sea Ecosystem overview is the latest addition to a collection which already includes the Barents Sea, Bay of Biscay and Iberian Coast, Celtic Seas, Greater North Sea, Icelandic Waters and the Norwegian Sea.
Eutrophication remains one of the major pressures on the Baltic ecosystem, having both direct and indirect impact. Photo: Eutrophication in the Baltic Sea, July 25, 2014. Photo: EOS-MODIS, NASA.